Many modern technological applications require precise frequency standards or clocks. For example, very precise navigational systems depend on clocks of extremely high accuracy. Atomic frequency standards form the basis for many such systems. One class of atomic beam standard that has found wide acceptance is based on a cesium beam tube. Cesium beam units are the present basis for most of the national standards of frequency and time. These standards are accurate to about a part in 10.sup.13.
Cesium beam standards utilize quantum effects arising from the nuclear magnetic hyperfine structure in the ground state of the cesium atom. The transition normally utilized arises from the electron-spin, nuclear-spin interaction. The transition in question is relatively insensitive to external influences such as electric and magnetic fields. This transition defines a frequency in the microwave region of the spectrum at 9,192,631,770 Hz.
Basically, the cesium beam tube provides an output that is very sensitive to the frequency of a microwave source that applies energy to the tube. The microwave source is tuned until the output of the tube is maximized. When this condition is satisfied, the frequency of the microwave source will be related in a known manner to the above-described transition frequency.
An ideal cesium beam tube operates as follows. A collimated beam of cesium atoms is passed through a magnetic state selector which selects cesium atoms in a first energy state. The selected atoms then traverse a microwave cavity in which the atoms absorb energy from or give energy to the microwave source. The absorbed or delivered energy causes some fraction of the atoms to make a transition to a second energy state. The number of atoms that made the transition is then determined in an analyzer. The frequency of the microwave source is continuously adjusted in a servo loop to maximize the output of the analyzer.
In prior art systems, the microwave frequency is generated by an RF chain which includes frequency multiplication of the output of a low-frequency voltage controlled oscillator. A 10 MHz crystal stabilized oscillator is often used. The servo loop operates on the low-frequency oscillator. The output of the low-frequency oscillator is typically split. Part of the output signal becomes the frequency standard which is provided by the system, the other part of the output signal is frequency multiplied and applied to a Cesium beam tube. This arrangement is dictated by lack of commercially practical crystal stabilized voltage controlled oscillators at frequencies in the 9 GHz region.
While the microwave frequency is closely correlated to the frequency of the above-mentioned transition, the frequency of the low frequency oscillator may fluctuate due to frequency pulling caused by variations in the loading of the frequency standard output. In addition, spurious signals from the circuit connected to this output can leak back into the atomic clock and corrupt the microwave signal by modulating it. In either case, frequency errors can result. To avoid these errors, isolation amplifiers are typically introduced between the signal splitting circuit and the external output. Unfortunately, such amplifiers can introduce a temperature sensitive phase shift which reduces the accuracy of the output during temperature changes, particularly when the output is being used as the basis of a high accuracy clock. Hence, a tradeoff is forced between errors caused by lack of isolation, and errors caused by side effects of having a lot of isolation.
A second problem with this prior art arrangement is the large difference between the system output and the microwave frequency. The large multiplication factor between the 10 MHz oscillator and the microwave generator reduces the degree of coherency between these two frequency sources. The multiplier circuits typically involve a number of tuned circuits having temperature sensitive components. Even with high stability inductors, the phase drift is significant. The phase drift in question increases with the degree of frequency multiplication. Ideally, the low-frequency oscillator should have a frequency closer to the 9 GHz microwave frequency, since a lower degree of multiplication would be needed.
A third problem with prior art cesium beam systems is the lack of multiple, well-isolated, phase coherent outputs. In principle, different output frequencies can be generated from the crystal stabilized oscillator by multiplying or dividing its frequency. However, the dividers introduce phase shift in the output signals depending on when the divider was started; hence, each time the frequency is shifted, a phase shift in the output signals, depending on when the divider is started, can be introduced. Such phase shifts are particularly troublesome in high precision clocks. In addition, only frequencies which are integer fractions of the 10 MHz oscillator frequency are obtained. Furthermore, there are a number of applications in which a standard frequency that is much greater than 10 MHz is desirable. To obtain such a standard frequency with prior art systems, frequency multiplication must be utilized. The same phase drift problems discussed above apply to such frequency multipliers.
Broadly, it is the object of the present invention to provide an improved RF chain for use in atomic clocks.
It is another object of the present invention to provide an RF chain that requires less frequency multiplication than prior art systems.
It is a further object of the present invention to provide an RF chain with improved output signal isolation without introducing temperature dependent phase shifts.
It is a still further object of the present invention to provide an RF chain having a selectable output signal frequency in which the output signal remains synchronized even when its frequency is shifted.
It is yet another object of the present invention to provide an RF chain that provides output frequencies above 10 MHz.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.